The present invention relates to cathode-ray tube circuits in general, and in particular to a geometry and focus correction circuit for a cathode-ray tube having an electromagnetic deflection system.
In a cathode-ray tube, hereinafter abbreviated CRT, having an electromagnetic deflection system, the relationship between deflection current through the deflection yoke and spot deflection over CRT faceplate is non linear principally as a result of two geometrically related phenomena. First, for electromagnetic deflection the sine of the angle .theta. of deflection is proportional to deflection current, or sin .theta.= kI. Second, the CRT faceplate is either flat or only partially curved rather than being spherical with the radial origin coincident with the point of beam deflection. Assuming observation from an infinite view point, a display image which is intended to be linear appears to be bowed toward the center of the screen. Such distortion is commonly referred to as pincushion distortion.
In addition to geometry distortion, defocusing of the spot occurs at the outer edges of a CRT display because the beam distance from the point of deflection to the faceplate varies as the beam moves away from the center of the screen. This is due to the electron velocity remaining constant after deflection and the focus point being at a fixed total path distance as the electron moves toward the screen.
Previous attempts to correct for pincushion distortion have included optical faceplate lenses, magnetic devices, and wave-shaping techniques. One of the most widely used correction methods is predistortion of the positioning signals, as the other methods are generally expensive, inflexible, and have a tendency to create other problems, such as non linearities.
A detailed analytic treatment of the predistortion required for correction of pincushion distortion was given by A. E. Popodi, "Linearity Correction for Magnetically Deflected Cathode-Ray Tubes," EDN Magazine, January 1964, pages 124-139. Popodi noted that if the deflection angle is known, the error in spot deflection may be determined. While providing mathematical equations describing pincushion distortion and the correction required, Popodi suggests a complex circuit comprising many diodes and resistors which provides a piece-wise linear approximation of the correction function. Subsequent investigators have suggested series-expansion approximation and complex circuits including analog-to-digital circuits to provide an approximation of the correction function.